Significant results were observed in both subjective (Tension–Anxiety subscale; p = 0.037) and objective measurements (Salivary CgA Concentrations; p = 0.034) for the intervention group after a single intake of REP hydrolysate.
However, no improvements were noted in some other subjective measurements (POMS 2 subscales and Euthymia Scale).
Additionally, daily intake for four weeks did not elicit any significant changes from baseline.
The results were based on a randomised, double-blind, and placebo-controlled trial conducted with 76 participants aged 20 to 64 years. They were randomly allocated to the intervention group (n = 38) and control group (n = 38).
The researchers wanted to investigate the antidepressant effect of REP hydrolysate – found to be significant in an animal model – on the human population. Two studies were conducted to evaluate the improvement in negative mood of humans after a single intake (Study 1) and daily for four weeks (Study 2).
REP is found in peeled rice commonly consumed across the world. The following rice oligo peptides found in REP hydrolysate, namely QQFLPEGQSQSQK, LPEGQSQSQK, and pEQFLPEGQSQSQK, accounted for the antidepressant-like effect found in the animal study.
“Even though various functionalities of REP have been reported, REP has not been provided much as a functional food because of its high dose (e.g., 10 g/day).
“Thus, turning REP into a functional ingredient that can be physiologically active with a small dose, namely functional peptides, would be a truly meaningful approach,” the paper stated regarding the food ingredient potential.
Participants were asked to self-rate their mood on the Profile of Mood Status 2nd edition for adults (POMS 2) and Euthymia Scale. For Study 2, the Insomnia Severity Index (ISI) was included as well.
Objective measurements were also taken, which include a health check and saliva sample to test for the concentration of chromogranin A (CgA).
In Study 1, participants had their measurements taken prior the intervention and three hours after consuming the test food. In Study 2, participants were measured prior the study, first day of the test food period and once a week during the four-week test food period.
The researchers settled for a four-week period as it was hypothesised to have significant intervention effect based on another clinical study on Gamma-aminobutyric acid (GABA) supplement on mood state.
The study was funded by funded by the Japan Science and Technology Agency and the Adaptable and Seamless Technology transfer Program.
The test food containing REP hydrolysate was supplied by Kameda Seika Co., Ltd. containing 25mg/each tablet, and two tablets were administered each day.
The placebo tablets were made indistinguishable in appearance and flavour, with comparable energy levels at 0.99 kcal for placebo and 0.98 kcal for test food.
No significant positive intervention for four-week intervention
The study posited that a four-week intake may be insufficient. It added that the regulatory pathways for REP hydrolysate to achieve an antidepressant-like effect vary for the short term (neural regulation) and long term (humoral regulation).
“Some food ingredients with mood-enhancing effects, which were implied to have humoral mechanisms, were tested for their effectiveness over a longer period.
“The effectiveness of saffron extract, one of the food ingredients that was reported to show an antidepression and an anxiolytic effect, was clinically confirmed with 8–12 weeks of intervention. Thus, for the humoral mechanism, it may require a longer intervention period than 4 weeks.”
Additionally, the researchers said that the design for Study 2 could have included stressor anticipation as well. Previous research reported that anticipating upcoming stressors also negatively affects mood.
“During Study 2, the participants were asked to self-evaluate their mood status over the past week, including the evaluation time point at which they were asked to rate their current mood in Study 1. Consequently, the dataset obtained on the subjective mood status from Study 2 can be translated to how retrospective stressors affected the current mood.”
The researchers concluded the paper suggesting REP hydrolysate as a “helpful and harmless ingredient that can be used to relax anxiety symptoms and nervousness and prevent future related symptoms.”
“The Effect of the Rice Endosperm Protein Hydrolysate on the Subjective Negative Mood Status in Healthy Humans: A Randomized, Double-Blind, and Placebo-Controlled Clinical Trial”
Authors: Nakayama, R., et al.